After the excitement of Larry Page and Larry Ellison testifying earlier this week, the Google-Oracle trial has turned into a highly technical discussion about Java.

So what the heck is really going on?

Here's our brief explainer.

Q: What is Java and why do these companies care?

A: Java is a programming language and platform created by Sun Microsystems back in the 1990s. It was originally designed for running apps on portable low-powered devices -- like phones -- but evolved to serve a bunch of other uses. The big promise of Java was "write once, run anywhere," although it didn't exactly work out that way for reasons too complicated to get into here.

Google uses some pieces of Java, and some spin-off technologies based on Java, in Android, the smartphone platform that's basically taking over the world.

Oracle thinks Google should have paid a license to use those pieces. Google disagrees.

The most interesting parts of the testimony so far have been on this issue -- like emails suggesting Google considered licensing Java from Sun before Oracle bought it.

Q: There's been a lot of headlines about 'APIs.' What the heck are they?

A: An API is an application programming interface. Every common platform has them. They're basically a recipe for programmers writing a piece of software to get the platform to do something, like draw a certain type of object on the screen. An API basically says "do this, and this should happen."

Without APIs, a computer platform would be useless.

Q: So why wouldn't Oracle want Google to use those APIs?

A. Java also includes class libraries, which are specific manifestations of those APIs. So they're basically the way that the Java platform actually accomplishes the task that the programmer has requested.

Class libraries are copyrightable, like any other software code.

Google did not ship any of the Sun Java class libraries in Android -- it created its own from an open-source spinoff of Java called Harmony.

The line between APIs and class libraries is a little vague, and this is where a lot of the testimony has been focused.

That's because even though it's POSSIBLE to write your own unique implementation of the same APIs, in practice it's very difficult. So applications usually end up depending on some of the specifics of the way the APIs were implemented.

Q: How many Java APIs did Google use?

A: 37.

Q. That's it?

A. Google also admits that it copied some lines of code from Sun Java into files used by Android. All of those lines of code were later removed.

Most of them were for test files, but approximately 9 lines of code did actually make it into a shipping version of Android. Google lawyer Robert Van Nest admitted it was a mistake, and should never have happened.

Android has more than 15,000 lines of source code, total.

And there are two patents that Oracle says Google infringes. Several other patent claims have already been thrown out.

Q. So that seems like a lot of fighting over a very small thing.

A. That's why the judge originally laughed Oracle's $6.1 billion damages claim out of court. If any damages are awarded in this case, they'll probably be much smaller. In the tens of millions of dollars, most likely.

Q. So what's this really about?

A. The original creator of Java, James Gosling, weighed in on this when the suit was first filed in 2010, saying this is mostly about "ego, money, and power."